Method of preparing electrodes for an electrochemical cell

ABSTRACT

A method of preparing an electrochemical cell wherein the electrode material adheres to the current collector to create good electrical contact is provided. A critical aspect in the process of preparing the polymer mixture for both the anode and cathode slurries is that the polymer (or copolymer) not be subject to high shear so as to be degraded. Polymer degradation contributes to the creation of the polymer concentration gradient in the electrode film.

FIELD OF THE INVENTION

The present invention relates to electrochemical devices and, moreparticularly, to non-aqueous electrochemical cells demonstratingimproved performance.

BACKGROUND OF THE INVENTION

Non-aqueous lithium electrochemical cells typically include an anode, alithium electrolyte prepared from a lithium salt dissolved in one ormore organic solvents and a cathode of an electrochemically activematerial, typically a chalcogenide of a transition metal. Duringdischarge, lithium ions from the anode pass through the liquidelectrolyte to the electrochemically active cathode material of thecathode where the ions are taken up with the simultaneous release ofelectrical energy. During charging, the flow of ions is reversed so thatlithium ions pass from the electrochemically active material through theelectrolyte and are plated back onto the lithium anode.

Recently, the lithium metal anode has been replaced with a carbon anodesuch as coke or graphite intercalated with lithium ions to form Li_(x)C. In operation of the cell, lithium passes from the carbon through theelectrolyte to the cathode where it is taken up just as in a cell with ametallic lithium anode. During recharge, the lithium is transferred backto the anode where it reintercalates into the carbon. Because nometallic lithium is present in the cell, melting of the anode does notoccur even under abuse conditions. Also, because lithium isreincorporated into the anode by intercalation rather than by plating,dendritic and spongy lithium growth does not occur. Non-aqueous lithiumelectrochemical cells are discussed in U.S. Pat. Nos. 4,472,487,4,668,595 and 5,028,500.

Current collectors are used with carbon anodes and cathodes asdescribed, for example, in U.S. Pat. Nos. 4,925,752 and 5,011,501. Infabricating cathodes for instance, a cathode material comprising asolvent, polymer, and electrochemically active particulate material islaminated to a current collector. Thereafter, the solvent is allowed toevaporate to form a sheet of cathode material on the current collector.Unfortunately, prior art anode and cathode materials often do not adhereadequately to the surface of the current collector which reduces theelectrochemical performance of the electrochemical cell.

SUMMARY OF THE INVENTION

The present invention is based in part on the discovery that improvedanodes and cathodes can be fabricated by increasing the homogeneity ofthe anode and cathode slurries and by minimizing the degree of polymerdegradation in the slurry during the mixing processes prior to casting.Anode and cathode slurries so prepared produce anode and cathodematerials which adhere well to the current collector thus creating goodelectrical contact.

In one aspect, the invention is directed to a method of preparing anelectrochemical cell comprising the steps of:

(a) preparing an anode mixture comprising a first polymer, anintercalation carbon material, and a first inert liquid solvent whereinthe first polymer is homogeneously distributed within said anodemixture;

(b) evaporating the first inert liquid solvent from the anode mixture toform an anode film;

(c) preparing a cathode mixture comprising a second polymer, a cathodeactive material, and a second inert liquid solvent wherein the secondpolymer is homogeneously distributed with said cathode mixture;

(d) evaporating the second liquid solvent from the cathode mixture toform a cathode film;

(e) forming an anode by laminating said anode film onto at least oneside of an anode current collector;

(f) forming a cathode by laminating said cathode film onto at least oneside of a cathode current collector;

(g) interposing a polymeric layer between said anode and cathode;

(h) placing an electrolyte solution comprising an electrolyte solventand an inorganic salt into said anode, cathode, and polymeric layer.

In another aspect, the invention is directed to a method of preparing anelectrochemical cell comprising the steps of:

(a) preparing an anode mixture comprising a first polymer, anintercalation carbon material, and a first inert liquid solvent whereinthe first polymer is homogeneously distributed within said anodemixture;

(b) coating the anode mixture onto at least one side of an anode currentconductor layer to form an anode;

(c) preparing a cathode mixture comprising a second polymer, a cathodeactive material, and a second inert liquid solvent wherein the secondpolymer is homogeneously distributed with said cathode mixture;

(d) coating the cathode mixture onto at least one side of a cathodecurrent collector layer to form a cathode;

(c) interposing a polymeric layer between said anode and cathode;

(d) removing said first and second inert liquid solvents; and

(e) placing an electrolyte solution comprising an electrolyte solventand an inorganic salt into said anode, cathode, and polymeric layer.

In yet another aspect, the invention is directed to an electrochemicalcell comprising:

an anode having anode current collector and a film of anode materiallaminated on at least one side of the anode current collector, whereinthe anode material comprises a first polymer that is homogeneouslydistributed in the film;

a cathode having cathode current collector and a film of cathodematerial laminated on at least one side of the cathode currentcollector, wherein the cathode material comprises a second polymer thatis homogeneously distributed in the film;

a polymeric layer interposed between the anode and cathode and whichcomprises an electrolyte solution comprising an electrolyte solvent andan inorganic salt.

A critical aspect in the process of preparing the polymer mixture forboth the anode and cathode slurries is that the polymer (or copolymer)not be subject to high shear so as to be degraded. It is believed thatpolymer degradation contributes to the creation of a polymerconcentration gradient in the electrode film. Degradation alsocontributes to poor physical property performance

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The present invention is directed to a method of preparing electrodessuitable for use in electrochemical devices. The invention is based inpart on the discovery that prior art casting techniques produceelectrodes wherein the polymer is not uniformly distributed therein. Itwas found that the solvent drying process caused polymer molecules to betransported toward the surface of the electrode slurry thereby creatinga polymer concentration gradient within the electrode. Specifically,after the solvent has evaporated, the polymer concentration is highestnear the exposed surface of the electrode film. Since the polymerfunctions in part to bind the electrode to the surface of the currentcollector, the result was poor adhesion and a reduction in electricalcontact. Furthermore, the concentration gradient within the electrodereduced the cohesiveness of the electrode itself.

Preferred cells include: a cathode comprising an active material, anintercalation based carbon anode, with each electrode capable ofreversibly incorporating (i.e., intercalating) an alkali metal ion, anda polymeric matrix containing an electrolyte solution comprising anorganic electrolyte solvent and a salt of the alkali metal. Eachelectrode has a current collector. Particularly preferredelectrochemical cells and batteries use lithium and salts thereof.

The anode of the present invention generally comprises an anode filmthat is laminated onto one or both sides of the current collector whichis a thin metal foil or grid. Typically, each anode film is from about100 μm to about 250 μm in thickness, preferably about 110 μm to about200 μm, and more preferably about 125 μm to about 175 μm

Similarly, the cathode of the present invention generally comprises acathode film that is laminated onto one or both sides of the currentcollector which is a thin metal foil or grid. Typically, each cathodefilm is from about 100 μm to about 200 μm in thickness, preferably about130 μm to about 175 μm, and more preferably about 140 μm to about 165μm.

The anode and cathode each also includes a current collector thatcomprises, for example, a screen, grid, expanded metal, woven ornon-woven fabric or knitted wire formed from an electron conductivematerial such as metals or alloys. Preferably, the current collector hasa thickness from about 25 μm to about 75 μm, preferably about 35 μm toabout 65 μm, and more preferably about 45 μm to about 55 μm. Eachcurrent collector is also connected to a current collector tab whichextends from the edge of the current collector. In batteries comprisingmultiple electrochemical cells, the anode tabs are preferably weldedtogether and connected to a nickel lead. The cathode tabs are similarlywelded and connected to a lead. External loads can be electricallyconnected to the leads. Current collectors and tabs are described inU.S. Pat. Nos. 4,925,752, 5,011,501, and 5,326,653, which areincorporated herein.

However, prior to describing this invention in further detail, thefollowing terms will be defined.

The term "plasticizer" refers to an organic solvent, with limitedsolubility of polymers, that facilitates the formation of porouspolymeric structures. By "porous structure" is meant that uponextraction of the plasticizer the polymer remains as a porous mass.Suitable plasticizers have high boiling points typically from about 100°C. to about 350° C. A number of criteria are important in the choice ofplasticizer including compatibility with the components of theelectrochemical cell precursor, processability, low polymer solubilityand extractability by dense gases. Preferred plasticizers include, forexample, dibutyl phthalate, dioctylphthalate, and acetates, glymes, andlow molecular weight polymers.

In operation, the plasticizer is first well mixed with a polymer.Thereafter the plasticizer is removed by extraction and in the processthe porous structure is formed. Preferably the weight ratio ofplasticizer to polymer is from about 1 to about 50, more preferablyabout 10 to about 30, and most preferably about 20 to about 25.

The term "electrochemical cell precursor" or "electrolytic cellprecursor" refers to the structure of the electrochemical cell prior tothe addition of the inorganic salt and electrolyte solution. Theprecursor typically comprises (each in precursor form) an anode, acathode, and polymeric matrix. The anode and/or cathode may each includea current collector.

The term "activation" refers to the placement of an inorganic salt andelectrolyte solvent into the porous portions of an electrochemical cellprecursor. After activation, the electrochemical cell is charged by anexternal energy source prior to use.

The term "electrolytic cell" or "electrochemical cell" refers to acomposite containing an anode, a cathode and an ion-conductingelectrolyte interposed therebetween.

The term "battery" refers to two or more electrochemical cellselectrically interconnected in an appropriate series/parallelarrangement to provide the required operating voltage and currentlevels.

The term "solid polymeric matrix" refers to an electrolyte compatiblematerial formed by polymerizing an inorganic or organic monomer (orpartial polymer thereof) and which, when used in combination with theother components of the electrolyte, renders the electrolyte solid.Suitable solid polymeric matrices are well known in the art and includesolid matrices formed from inorganic polymers, organic polymers or amixture of organic polymers with inorganic non-polymeric materials.Preferably, the solid polymeric matrix is an organic matrix derived froma solid matrix forming monomer and from partial polymers of a solidmatrix forming monomer. See, for example, U.S. Pat. No. 4,925,751, whichis incorporated herein.

Alternatively, the solid polymeric matrix can be used in combinationwith a non-polymeric inorganic matrix. See, for example, U.S. Pat. No.4,990,413, which is incorporated herein by reference in its entirety.Suitable non-polymeric inorganic materials for use in conjunction withthe solid polymeric matrix include, by way of example, β-alumina, silveroxide, lithium iodide, and the like. Suitable inorganic monomers arealso disclosed in U.S. Pat. Nos. 4,247,499, 4,388,385, 4,414,607,4,394,280, 4,432,891, 4,539,276, and 4,557,985 each of which isincorporated herein by reference.

The term "a solid matrix forming monomer" refers to inorganic or organicmaterials which in monomeric form can be polymerized, preferably in thepresence of an inorganic ion salt, and a solvent mixture of an organiccarbonate and a glyme compound, to form solid matrices which aresuitable for use as solid electrolytes in electrolytic cells. Suitablesolid matrix forming monomers are well known in the an and theparticular monomer employed is not critical. The solid matrix formingmonomers may also comprise heteroatoms capable of forming donor acceptorbonds with inorganic cations (e.g., alkali ions).

Examples of suitable organic solid matrix forming monomers include, byway of example, propylene oxide, ethyleneimine, ethylene oxide,epichlorohydrin, acryloyl-derivatized polyalkylene oxides (as disclosedin U.S. Pat. No. 4,908,283), urethane acrylate, vinyl sulfonatepolyalkylene oxides (as disclosed in U.S. Pat. No. 5,262,253, whichpatent is incorporated herein by reference in its entirety), and thelike as well as mixtures thereof. Examples of suitable inorganic solidmatrix forming monomers include, by way of example, phosphazenes andsiloxanes. Phosphazene monomers and the resulting polyphosphazene solidmatrix are disclosed by Abraham et al., Proc. Int. Power Sources Symp.,34th, pp. 81-83 (1990) and by Abraham et al., J. ElectrochemicalSociety, Vol. 138, No. 4, pp. 921-927 (1991).

The term "a partial polymer of a solid matrix forming monomer" refers tosolid matrix forming monomers which have been partially polymerized toform reactive oligomers. Partial polymerization may be conducted for thepurpose of enhancing the viscosity of the monomer, decreasing thevolatility of the monomer, and the like. Partial polymerization isgenerally permitted so long as the resulting partial polymer can befurther polymerized, preferably in the presence of a solvent, such as, amixture of organic carbonate(s) to form solid polymeric matrices whichare suitable for use as solid electrolytes in electrolytic cells.

The term "cured" or "cured product" refers to the treatment of the solidmatrix forming monomer or partial polymer thereof under polymerizationconditions (including cross-linking) so as to form a solid polymericmatrix. Suitable polymerization conditions are well known in the art andinclude by way of example, heating the monomer, irradiating the monomerwith UV light, electron beams, and the like. The resulting cured productpreferably contains repeating units containing at least one heteroatomsuch as oxygen or nitrogen which is capable of forming donor acceptorbonds with inorganic cations (alkali ions). Examples of suitable curedproducts suitable for use in this invention are set forth in U.S. Pat.Nos. 4,830,939 and 4,990,413 which are incorporated herein by referencein their entirety.

The solid matrix forming monomer or partial polymer can be cured orfurther cured prior to or after addition of the salt, solvent and,optionally, a viscosifier. For example, a composition comprisingrequisite amounts of the monomer or partial polymer, salt, organiccarbonate solvent and viscosifier can be applied to a substrate and thencured. Alternatively, the monomer or partial polymer can be first curedand then dissolved in a suitable volatile solvent. Requisite amounts ofthe salt, organic carbonate solvent and viscosifier can then be added.The mixture is then placed on a substrate and cured; removal of thevolatile solvent would result in the formation of a solid electrolyte.In either case, the resulting solid electrolyte would be a homogeneous,single phase product which is maintained upon curing, and does notreadily separate upon cooling to temperatures below room temperature.

Alternatively, the solid polymeric matrix can be formed by a castingprocess which does not require the use of monomers or prepolymers, thatis, no curing is required. A preferred method employs a copolymer ofpolyvinylidenedifluroide and hexafluoropropylene dissolved in acetone orother suitable solvent. Upon casting the solution, the solvent isevaporated to form the solid polymeric matrix. The solution may becasted directly onto a current collector. Alternatively, the solution iscasted onto a substrate, such as a carrier web, and after the solvent(e.g., acetone) is removed, an electrode film is formed thereon.

The term "salt" refers to any salt, for example, an inorganic salt,which is suitable for use in a non-aqueous electrolyte. Representativeexamples of suitable inorganic ion salts are alkali metal salts of lessmobile anions of weak bases having a large anionic radius. Examples ofsuch anions are I⁻, Br⁻, SCN⁻, ClO₄ ⁻, BF₄ ⁻, PF₆ ⁻, AsF₆ ⁻, CF₃ COO⁻,CF₃ SO₃ ⁻, N(SO₂ CF₃)₂ ⁻, and the like. Specific examples of suitableinorganic ion salts include LiClO₄, LiSCN, LiBF₄, LiAsF₆, LiCF₃ SO₃,LiPF₆, (CF₃ SO₂)₂ NLi, (CF₃ SO₂)₃ CLi, NaSCN, and the like. Theinorganic ion salt preferably contains at least one cation selected fromthe group consisting of Li, Na, Cs, Rb, Ag, Cu, Mg and K.

The electrolyte typically comprises from about 5 to about 25 weightpercent of the inorganic ion salt based on the total weight of theelectrolyte; preferably, from about 10 to 20 weight percent; and evenmore preferably from about 10 to about 15 weight percent. The percentageof salt depends on the type of salt and electrolytic solvent employed.

The term "compatible electrolyte solvent" or "electrolytic solvent," orin the context of components of the non-aqueous electrolyte, just"solvent," is a low molecular weight organic solvent added to theelectrolyte and/or the cathode composition, which may also serve thepurpose of solvating the inorganic ion salt. The solvent is anycompatible, relatively non-volatile, aprotic, relatively polar, solvent.Preferably, these materials have boiling points greater than about 85°C. to simplify manufacture and increase the shelf life of theelectrolyte/battery. Typical examples of solvent are mixtures of suchmaterials as dimethyl carbonate, diethyl carbonate, propylene carbonate,ethylene carbonate, methyl ethyl carbonate, gamma-butyrolactone,triglyme, tetraglyme, dimethylsulfoxide, dioxolane, sulfolane, and thelike. When using propylene carbonate based electrolytes in anelectrolytic cell with graphite anodes, a sequestering agent, such as acrown ether, is added in the electrolyte.

For electrochemical cells where (1) the cathode comprises lithiatedcobalt oxides, lithiated manganese oxides, lithiated nickel oxides,Li_(x) Ni_(1-y) Co_(y) O₂, where x is preferably about 1 and y ispreferably 0.1-0.9, LiNiVO₄, or LiCoVO₄, and (2) the anode comprisescarbon, the electrolytic solvent preferably comprises a mixture ofethylene carbonate and dimethyl carbonate. For electrochemical cellswhere the cathode comprises vanadium oxides, e.g., V₆ O₁₃ and the anodeis lithium, the electrolytic solvent preferably comprises a mixture ofpropylene carbonate and triglyme.

The term "organic carbonate" refers to hydrocarbyl carbonate compoundsof no more than about 12 carbon atoms and which do not contain anyhydroxyl groups. Preferably, the organic carbonate is an aliphaticcarbonate and more preferably a cyclic aliphatic carbonate.

Suitable cyclic aliphatic carbonates for use in this invention include1,3-dioxolan-2-one (ethylene carbonate); 4-methyl-1,3-dioxolan-2-one(propylene carbonate); 4,5-dimethyl-1,3-dioxolan-2-one;4-ethyl-1,3-dioxolan-2-one; 4,4-dimethyl-1,3-dioxolan-2-one;4-methyl-5-ethyl-1,3-dioxolan-2-one; 4,5-diethyl-1,3-dioxolan-2-one;4,4-diethyl-1,3-dioxolan-2-one; 1,3-dioxan-2-one;4,4-dimethyl-l,3-dioxan-2-one; 5,5-dimethyl-1,3-dioxan-2-one;5-methyl-1,3-dioxan-2-one; 4-methyl-1,3-dioxan-2-one;5,5-diethyl-1,3-dioxan-2-one; 4,6-dimethyl-1,3-dioxan-2-one;4,4,6-trimethyl-1,3-dioxan-2-one; and spiro(1,3-oxa-2-cyclohexanone-5',5',1',3'-oxa-2'-cyclohexanone).

Several of these cyclic aliphatic carbonates are commercially availablesuch as propylene carbonate and ethylene carbonate. Alternatively, thecyclic aliphatic carbonates can be readily prepared by well knownreactions. For example, reaction of phosgene with a suitablealkane-α,β-diol (dihydroxy alkanes having hydroxyl substituents onadjacent carbon atoms) or an alkane-α,γ-diol (dihydroxy alkanes havinghydroxyl substituents on carbon atoms in a 1,3 relationship) yields an acyclic aliphatic carbonate for use within the scope of this invention.See, for instance, U.S. Pat. No. 4,115,206, which is incorporated hereinby reference in its entirety.

Likewise, the cyclic aliphatic carbonates useful for this invention maybe prepared by transesterification of a suitable alkane-α,β-diol or analkane-α,γ-diol with, e.g., diethyl carbonate under transesterificationconditions. See, for instance, U.S. Pat. Nos. 4,384,115 and 4,423,205which are incorporated herein by reference in their entirety. Additionalsuitable cyclic aliphatic carbonates are disclosed in U.S. Pat. No.4,747,850 which is also incorporated herein by reference in itsentirety.

The term "viscosifier" refers to a suitable viscosifier for solidelectrolytes. Viscosifiers include conventional viscosifiers such asthose known to one of ordinary skill in the art. Suitable viscosifiersinclude film forming agents well known in the art which include, by wayof example, polyethylene oxide, polypropylene oxide, copolymers thereof,and the like, having a number average molecular weight of at least about100,000, polyvinylpyrrolidone, carboxymethylcellulose, and the like.Preferably, the viscosifier is employed in an amount of about 1 to about10 weight percent and more preferably at about 2.5 weight percent basedon the total weight of the electrolyte composition.

The anode typically comprises a compatible anodic material which is anymaterial which functions as an anode in a solid electrolytic cell. Suchcompatible anodic materials are well known in the art and include, byway of example, lithium, lithium alloys, such as alloys of lithium withaluminum, mercury, manganese, iron, zinc, intercalation based anodessuch as those employing carbon, tungsten oxides, and the like. Preferredanodes include lithium intercalation anodes employing carbon materialssuch as graphite, cokes, mesocarbons, and the like. Such carbonintercalation based anodes typically include a polymeric binder andextractable plasticizer suitable for forming a bound porous compositehaving a molecular weight of from about 1,000 to 5,000,000. Examples ofsuitable polymeric binders include EPDM (ethylene propylene diaminetermonomer), PVDF (polyvinylidene difluoride), EAA (ethylene acrylicacid copolymer), EVA (ethylene vinyl acetate copolymer), EAA/EVAcopolymers, and the like. The anode may also include an electronconducting material such as carbon black.

The cathode typically comprises a compatible cathodic material (i.e.,insertion compounds) which is any material which functions as a positivepole in a solid electrolytic cell. Such compatible cathodic materialsare well known in the art and include, by way of example, transitionmetal oxides, sulfides, and selenides, including lithiated compoundsthereof. Representative materials include cobalt oxides, manganeseoxides, molybdenum oxides, vanadium oxides, sulfides of titanium,molybdenum and niobium, the various chromium oxides, copper oxides,lithiated cobalt oxides, e.g., LiCoO₂ and LiCoVO₄, lithiated manganeseoxides, e.g., LiMn₂ O₄, lithiated nickel oxides, e.g., LiNiO₂ andLiNiVO₄, and mixtures thereof. Cathode-active material blends of Li_(x)Mn₂ O₄ (spinel) is described in U.S. Pat. No. 5,429,890 which isincorporated herein. The blends can include Li_(x) Mn₂ O₄ (spinel) andat least one lithiated metal oxide selected from Li_(x) NiO₂ and Li_(x)CoO₂ wherein 0<x≦2. Blends can also include Li_(y) -α-MnO₂ (0≦y<1) whichis Li_(y) NH₄ Mn₈ O₁₆ (0≦y<1) which has a hollandite-type structure.Li_(y) -α-MnO₂ where 0≦y<0.5 is preferred. αMnO₂ can be synthesized byprecipitation from a reaction between a MnSO₄ solution and (NH₄)₂ S₂ O₈as an oxidizing agent.

Lithiation (also referred to as "prelithiation) of αMnO₂ can beaccomplished via a solid state reaction:

    NH.sub.4 Mn.sub.8 O.sub.16 +LiOH→LiMnO.sub.2 (400° C.).

Li-α-MnO₂ retains the same structure as Hollandite. See, Botkovitz et.al., J. of Power Sources, 43-44 (1993) 657-665, which is incorporatedherein, for a discussion of the structure and electrochemicalcharacteristics of Li-α-MnO₂. Li_(y) -α-MnO₂ 0≦y<0.5 is commerciallyavailable from SEDEMA, Tertre, Belgium.

In one preferred embodiment, the compatible cathodic material is mixedwith an electroconductive material including, by way of example,graphite, powdered carbon, powdered nickel, metal particles, conductivepolymers (i.e., characterized by a conjugated network of double bondslike polypyrrole and polyacetylene), and the like, and a polymericbinder to form under pressure a positive cathodic plate. Suitablebinders for use in the cathode have a molecular weight of from about1,000 to 5,000,000.

In one preferred embodiment, the cathode is prepared from a cathodepaste which comprises from about 35 to 65 weight percent of a compatiblecathodic material; from about 1 to 20 weight percent of anelectroconductive agent; from about 1 to 20 weight percent of suitablepolymeric binders that may include EPDM (ethylene propylene dienetermonomer), PVDF (polyvinylidene difluoride), EAA (ethylene acrylicacid copolymer), EVA (ethylene vinyl acetate copolymer), EAA/EVAcopolymers, and the like; from about 0 to about 20 weight percent ofpolyethylene oxide having a number average molecular weight of at least100,000; from about 10 to 50 weight percent of solvent comprising a 10:1to 1:4 (w/w) mixture of an organic carbonate and a glyme; and from about5 weight percent to about 25 weight percent of a sold matrix formingmonomer or partial polymer thereof. Also included is an ion conductingamount of an inorganic ion salt. Generally, the amount of the salt isfrom about 1 to about 25 weight percent. (All weight percents are basedon the total weight of the cathode.)

The electrolyte composition typically comprises from about 5 to about 25weight percent of the inorganic ion salt based on the total weight ofthe electrolyte; preferably, from about 10 to 20 weight percent; andeven more preferably from about 10 to about 15 weight percent. Thepercentage of salt depends on the type of salt and electrolytic solventemployed.

The electrolyte composition typically comprises from 0 to about 80weight percent electrolyte solvent (e.g., organic carbonate/glymemixture) based on the total weight of the electrolyte; preferably fromabout 60 to about 80 weight percent; and even more preferably about 70weight percent.

The electrolyte composition typically comprises from about 5 to about 30weight percent of the solid polymeric matrix based on the total weightof the electrolyte; preferably from about 15 to about 25 weight percent.

In a preferred embodiment, the electrolyte composition further comprisesa small amount of a film forming agent. Suitable film forming agents arewell known in the art and include, by way of example, polyethyleneoxide, polypropylene oxide, copolymers thereof, and the like, having anumbered average molecular weight of at least about 100,000. Preferably,the film forming agent is employed in an amount of about 1 to about 10weight percent and more preferably at about 2.5 weight percent based onthe total weight of the electrolyte composition.

Methodology

Electrochemical cells are known in the art. See, for example, U.S. Pat.Nos. 5,300,373, 5,316,556, 5,346,385, 5,262,253, 4,472,487, 4,668,595,and 5,028,500, all of which are incorporated herein. The inventivemethod can be adapted to form anode, cathode, and/or polymeric matrixstructures in prior art electrochemical cells. The following illustratesa method of how an electrolytic cell could be fabricated with theinventive process. Examples 1 and 2 describe the process of preparingthe anode and cathodes, respectively. Example 3 describes the proceduresfor fabricating a solid electrolytic cell.

The invention will be described using the anode and cathode structureswherein electrode materials (or films) are laminated onto both sides ofthe current collectors, however, it is understood that the invention isapplicable to other configurations, for example, where one side of theanode and/or cathode current collector is laminated.

EXAMPLE 1

The anode current collector employed was a sheet of expanded coppermetal that is about 50 μm thick. It is available under the designationDelker #2 Cu5-125 (flatten) Delker, Branford, Conn.

The anode slurry was prepared as follows:

A polymer mixture comprising a copolymer of polyvinylidenedifluoride(PVDF) and hexafluoropropylene (HFP) was prepared by mixing 23.3 gramsof the copolymer in 100 ml of acetone. The copolymer (ave. meltviscosity 23,000-27,000 poise) was Kynar Flex 2801™ from Elf AtochenNorth America, Philadelphia, Pa. The mixture was stirred for about 24hours in a milling jar available from VWR Scientific, San Francisco,Calif., catalogue No. H-04172-00. The copolymer functions as a binderfor the carbon in the anode.

A graphite mixture was prepared separately by first adding 80 grams ofgraphite into 3.1 grams of carbon black into a solution containing 200grams acetone, 36 grams dibutyl phthalate, and 0.5 grams of asurfactant. A preferred graphite comprises a 50:50 (by weight) mixtureof a synthetic graphite available under the designation SFG-15™ fromLonza G & T, Ltd. (Sins, Switzerland) and graphitized mesocarbonmicrobeads available under the designation MCMB25-28™ from Osaka GasCo., Japan. A preferred carbon black is available under the designationSuper P™ from M.M.M. Carbon, Willbroeck, Belgium. Preferred surfactantsinclude pluronic FC68™ from BASF, Mr. Olive, N.J. and Fluorad 430™ from3M Co. St. Paul, Minn. The surfactant disperses the graphite. Thegraphite mixture was then vigorously mixed in a high shear mixer until asubstantially homogeneous blend was formed. A suitable mixer isavailable from Ross, Model ME100DLX, Hauppauge, N.Y. operating at a highsetting (about 10,000 RPM) for 30 minutes.

The anode slurry was prepared by mixing the polymer mixture and thegraphite mixture together under low shear conditions to form the anodeslurry wherein the components are well mixed. A portion of the acetoneis allowed to evaporate from the slurry before it was laminated ontoeach side of the current collector. Anode films form when the remainingportion of the acetone evaporates.

EXAMPLE 2

The cathode current collector employed was a sheet of expanded aluminumthat is about 50 μm thick. The aluminum grid is available under thedesignation 2AL5-077 from Delker, Brandford, Conn.

The cathode slurry was prepared as follows:

A polymer mixture comprising a copolymer of polyvinylidenedifluoride(PVDF) and hexafluoropropylene (HFP) was prepared by mixing 26.7 gramsof the copolymer in 100 grams of acetone. The copolymer was Kynar Flex2801™. The mixture was stirred for about 24 hours in a milling jar.

An active material mixture was prepared separately by first adding 173.4grams of a cathode-active material blend of Li_(x) Mn₂ O₄ (spinel)(0<x≦2) and Li_(y) -α-MnO₂ (0≦y<1) (1:1 weight ratio), 14.7 grams ofcarbon black (Super P™) into a solution containing 333.3 grams acetone,51.9 grams dibutyl phthalate, and 0.9 grams of a surfactant. The mixturewas then vigorously mixed in the a high shear mixer until asubstantially homogeneous blend was formed.

The cathode slurry was prepared by mixing the polymer mixture and theLi_(x) Mn₂ O₄ mixture together under low shear conditions to form theanode slurry wherein the components are well mixed. A portion of theacetone is allowed to evaporate from the slurry before it was laminatedonto each side of the current collector. Cathode films form when theremaining portion of the acetone evaporates.

The above anode and cathode films were formed directly on the currentcollector by laminating the slurry mixtures onto the current collectorsurfaces. Alternatively, each film can be prepared by first casting aslurry onto a substrate or carrier web and allowing the solvent toevaporate thus leaving the film. Thereafter, the films can be laminatedonto each side of the current collector.

EXAMPLE 3

A solid electrochemical cell is prepared by first positioning apolymeric matrix between the anode and cathode and thereafter fusing thestructures under moderate pressure and temperature (e.g., 130° C.) toform an electrochemical cell precursor. The polymeric matrix is formedby casting a polymeric slurry comprising acetone, dibutyl phthalate,silanized fumed SiO₂, and the PVDF/HFP copolymer on a suitable substrateor carrier web and allowing the acetone to evaporate. No curing byradiation is required. The SiO₂ is a filler which impart toughness andstrength to the film. In addition, it is believed that the SiO₂ assiststhe activation process by creating phyico-chemical conditions such thatthe electrolyte solution quickly and completely fills the pores createdby the extraction of the dibutyl phthalate. Preferably, the polymericslurry is mixed under low shear conditions as not to degrade thecopoloymer. The precursor is then packaged in moisture-impermeablematerial which is described, for example, in U.S. Pat. No. 5,326,653which is incorporated herein.

A critical aspect in the process of preparing the polymer mixture forboth the anode and cathode slurries is that the polymer (or copolymer)not be subject to high shear so as to be degraded. It is believed thatpolymer degradation contributes to the creation of the polymerconcentration gradient in the electrode film. Furthermore, preferablythe polymer or copolymer employed has a high average molecular weight.Preferably the average molecular weight is between 50K to 750K, morepreferably 200K to 500K, and most preferably 250K to 300K. Furthermore,it is preferred that polymer or copolymer has a narrow molecular weightrange. Preferably M_(n) /M_(w) ≅1.0.

Next, the dibutyl phthalate plasticizer is extracted from the precursorbefore being activated. Extraction can be accomplished usingconventional organic liquid solvents such as diethyl ether or by a densefluid or gas which refers to a gas compressed and heated to eithersupercritical or subcritical conditions to achieve liquid-likedensities. Dense gases and fluids are known in the art. See, forexample, U.S. Pat. Nos. 5,013,366, 5,267,455, 4,219,333, 4,012,194, and3,969,196, which are incorporated herein. A preferred dense gas iscarbon dioxide. Following extraction, the precursor is firstpre-packaged in moisture impermeable material described, for example, inU.S. Pat. No. 5,326,653 which is incorporated herein and then activated.Activation preferably takes place in an inert (e.g., argon) atmosphere.Finally, the packaging is sealed and the electrochemical cell is readyfor use.

EXPERIMENTAL

Sample electrode films were prepared by coating electrode slurries ontodifferent substrate surfaces and the resulting electrode films weresubjected to EDX analyses. The electrodes slurries initially comprised58.4% acetone, 10.5% dibutyl phthalate, 6.8% polyvinylidene fluoride(ave. MW 120K), and 24.3% graphite, with all percentages by weight. ForExample 4, the slurries were mixed together in a Waring Blender underhigh shear conditions. Specifically, acetone and dibutyl phthalate wereadded to the stainless steel container. Then approximately 1/3 of thegraphite was added and the blender turned on for approximately 30seconds. The remaining graphite was added in 2 sequential additionsfollowed by 30 seconds of mixing. After the last addition, the mixturewas allowed to mix for 30 minutes, at the lowest level of blender speed(approximately 9K RPM). Thereafter, the slurries were casted onto aglass substrate and the acetone was allowed to evaporate under ambientconditions. For Example 5, the slurry was prepared by the inventivemethod wherein the polymer mixture was blended separately under lowshear conditions.

EDX analyses were performed with a 10 KV electron beam with a Cambridgemodel 360 Stereoscan SEM equipped with PGT-4Plus EDX analyzer (withlight element detection capability). Carbon and fluorine ratios wereobtained from the top (exposed to atmosphere) and bottom (in contactwith substrate) surfaces and from several other locations in thelongitudinal cross-sections of the films. Each EDX spectrum was acquiredfor 100 sec. and then the background corrected, integrated counts withinthe C and F peaked used to compute the C/F ratios. Since the only sourceof fluorine is the polymer, the C/F ratio indicates the relative polymerconcentration in the film at the location of analysis. The higher theC/F ratio, the lower the polymer concentration.

As is apparent, in Example 4 the C/F ratio was lowest at the top surfaceand highest at the bottom surface. This suggests that a significantpolymer concentration gradient was present within these films. Theconcentration gradient was not seen in the film of Example 5. Thesubsequent extraction of the plasticizer does not affect theconcentration gradient.

Based on this comparative data, it is believed that with the inventiveprocess electrode (both anode and cathode) films can be fabricatedwherein the polymer concentration at the top and bottom surfaces issubstantially the same.

EXAMPLE 4

    ______________________________________                                        total film thickness is 210.0 μm                                           analysis location                                                                          distance from top (μM)                                                                     C/F ratio                                        ______________________________________                                        top surface  --              3.48                                             A            5.0             5.54                                             B            28.3            7.75                                             C            55.7            8.12                                             D            85.6            8.46                                             E            115.0           9.72                                             F            141.0           10.30                                            G            170.0           10.44                                            H            203.0           11.15                                            bottom surface                                                                             --              12.30                                            ______________________________________                                    

EXAMPLE 5

    ______________________________________                                        total film thickness is 100.0 μm                                           analysis location                                                                          distance from top (μM)                                                                     C/F ratio                                        ______________________________________                                        top surface  --                                                               A            5.0             6.57                                             B            27.1            7.36                                             C            49.1            7.67                                             D            68.1            8.05                                             E            94.6            6.69                                             bottom surface                                                                             --              6.41                                             ______________________________________                                    

While the invention has been described in terms of various preferredembodiments, the skilled artisan will appreciate the variousmodifications, substitutions, and changes which may be made withoutdeparting from the spirit hereof. The descriptions of the subject matterin this disclosure are illustrative of the invention and are notintended to be construed as limitations upon the scope of the invention.

What is claimed is:
 1. A method of preparing an electrochemical cellcomprising the steps of:(a) preparing an anode mixture comprising afirst polymer, an intercalation carbon material, and a first inertliquid solvent wherein the first polymer is homogeneously distributedwithin said anode mixture; (b) evaporating the first inert liquidsolvent from the anode mixture to form an anode film; (c) preparing acathode mixture comprising a second polymer, a cathode active material,and a second inert liquid solvent wherein the second polymer ishomogeneously distributed with said cathode mixture; (d) evaporating thesecond liquid solvent from the cathode mixture to form a cathode film;(e) forming anode by laminating said anode film onto at least one sideof an anode current collector; (f) forming a cathode by laminating saidcathode film onto at least one side of a cathode current collector; (g)interposing a polymeric layer between said anode and cathode; (h)placing an electrolyte solution comprising an electrolyte solvent and aninorganic salt into said anode, cathode, and polymeric layer.
 2. Themethod of claim 1 wherein the anode mixture, cathode mixture, andpolymeric layer each further comprises a plasticizer and said methodfurther comprises the step of removing said plasticizer before step (e).3. The method of claim 1 wherein the step of preparing the anode mixturecomprises the steps of:(i) mixing the first polymer and a portion ofsaid first inert liquid solvent under low shear conditions; (ii) mixingthe intercalation carbon material and a portion of said first inertliquid solvent; and (iii) mixing the mixtures of steps (i) and (ii)under low shear conditions; and wherein the step of preparing thecathode mixture comprises the steps of: (iv) mixing the first polymerand a portion of said second inert liquid solvent under low shearconditions; (v) mixing the cathode active material and a portion of saidfirst inert liquid solvent; and (vi) mixing the mixtures of steps (iv)and (v) under low shear conditions.
 4. The method of claim 1 wherein theanode mixture, cathode mixture, and polymeric layer each furthercomprises a plasticizer and said method further comprises the step ofremoving said plasticizer prior to step (h), wherein the step ofpreparing the anode mixture comprises the steps of:(i) mixing the firstpolymer and a portion of said first inert liquid solvent under low shearconditions; (ii) mixing the intercalation carbon material, plasticizer,and a portion of said first inert liquid solvent; and (iii) mixing themixtures produced in steps (i) and (ii) under low shear conditions; andwherein the step of preparing the cathode mixture comprises the stepsof: (iv) mixing the first polymer and a portion of said second inertliquid solvent under low shear conditions; (v) mixing the cathode activematerial, plasticizer, and a portion of said first inert liquid solvent;and (vi) mixing the mixtures produced in steps (iv) and (v) under lowshear conditions.
 5. The method of claim 1 wherein said first and secondpolymers comprise polyvinylidenedifluoride.
 6. The method of claim 1wherein said first and second solvents comprise acetone.
 7. The methodof claim 2 wherein said plasticizer comprises dibutyl phthalate.
 8. Themethod of claim 1 wherein each of said anode mixture and cathode mixturefurther comprise an electrically conductive material and a surfactant.9. The method of claim 1 wherein the polymeric layer comprises polymericmatrix and third inert liquid solvent, said third inert liquid solventbeing removed prior to step (h).
 10. A method of preparing anelectrochemical cell comprising the steps of:(a) preparing an anodemixture comprising a first polymer, an intercalation carbon material,and a first inert liquid solvent wherein the first polymer ishomogeneously distributed within said anode mixture; (b) coating theanode mixture onto at least one side of an anode current conductor layerto form an anode; (c) preparing a cathode mixture comprising a secondpolymer, a cathode active material, and a second inert liquid solventwherein the second polymer is homogeneously distributed with saidcathode mixture; (d) coating the cathode mixture onto at least one sideof a cathode current collector layer to form a cathode; (c) interposinga polymeric layer between said anode and cathode; (d) removing saidfirst and second inert liquid solvents; and (e) placing an electrolytesolution comprising an electrolyte solvent and an inorganic salt intosaid anode, cathode, and polymeric layer.
 11. The method of claim 10wherein the anode mixture, cathode mixture, and polymeric layer eachfurther comprises a plasticizer and said method further comprises thestep of removing said plasticizer before step (e).
 12. The method ofclaim 10 wherein the step of preparing the anode mixture comprises thesteps of:(i) mixing the first polymer and a portion of said first inertliquid solvent under low shear conditions; (ii) mixing the intercalationcarbon material and a portion of said first inert liquid solvent; and(iii) mixing the mixtures of steps (i) and (ii) under low shearconditions; and wherein the step of preparing the cathode mixturecomprises the steps of: (iv) mixing the first polymer and a portion ofsaid second inert liquid solvent under low shear conditions; (v) mixingthe cathode active material and a portion of said first inert liquidsolvent; and (vi) mixing the mixtures of steps (iv) and (v) under lowshear conditions.
 13. The method of claim 10 wherein the anode mixture,cathode mixture, and electrolyte composition each further comprises aplasticizer and said method further comprises the step of removing saidplasticizer prior to step (e), wherein the step of preparing the anodemixture comprises the steps of:(i) mixing the first polymer and aportion of said first inert liquid solvent under low shear conditions;(ii) mixing the intercalation carbon material, plasticizer, and aportion of said first inert liquid solvent; and (iii) mixing themixtures produced in steps (i) and (ii) under low shear conditions; andwherein the step of preparing the cathode mixture comprises the stepsof: (iv) mixing the first polymer and a portion of said second inertliquid solvent under low shear conditions; (v) mixing the cathode activematerial, plasticizer, and a portion of said first inert liquid solvent;and (vi) mixing the mixtures produced in steps (iv) and (v) under lowshear conditions.
 14. The method of claim 10 wherein said first andsecond polymers comprise polyvinylidenedifluoride.
 15. The method ofclaim 10 wherein said first and second solvents comprise acetone. 16.The method of claim 11 wherein said plasticizer comprises dibutylphthalate.
 17. The method of claim 10 wherein each of said anode mixtureand cathode mixture further comprise an electrically conductive materialand a surfactant.
 18. The method of claim 10 wherein the polymeric layercomprises polymeric matrix and third inert liquid solvent, said thirdinert liquid solvent being removed prior to step (e).
 19. The method ofclaim 1 wherein said first and second polymers comprise a copolymer ofpolyvinylidenedifluoride and hexafluoropropylene.
 20. The method ofclaim 10 wherein said first and second polymers comprise a copolymer ofpolyvinylidenedifluoride and hexafluoropropylene.